In 1918, the Lewis estate including the castle was bought by industrialist Lord Leverhulme from the Matheson family. He gifted the castle to the people of Stornoway parish in 1923. During World War II the Castle was taken over as accommodation for air and ground crew of 700 Naval Air Squadron, who operated a detachment of six Supermarine Walrus aircraft from a slipway at Cuddy Point in the Grounds. The base was referred to as HMS Mentor. After the war, the Castle was also used for accommodation for students of Lews Castle College in the 1950s.